The present invention generally relates to coextrusion of polymer articles from two or more melt streams comprising one or more types of polymers. The invention more particularly relates to coextrusion of polymer articles having a multiplicity of layers or microlayers of the same or different polymer materials.
In the production of articles such as polymer films, it has been recognized that improvements in one or more properties of a polymer article can be achieved by manufacturing the article as a layered structure having two or more layers of the same or different polymer materials. For example, one polymer material may be good in property A but poor in property B, while another polymer material may be weak in property A but may excel in property B. An article formed with layers of both polymer materials can achieve good performance in both properties A and B.
It has also been found that a laminar morphology can be advantageous even when a single type of polymer material makes up the polymer article. Thus, a polymer article can be formed of multiple layers of the same polymer material. Each layer is individually extruded and the layers are combined one atop another. Such a laminar structure can attain properties that an equivalent single-layer structure cannot.
Coextrusion is an advantageous method for producing such layered articles because the various layers are formed substantially simultaneously and interlayer bonding is achieved during the coextrusion of the layers while the polymer materials are still in a molten state. The coextrusion process thus is streamlined and efficient. In a simple “static” coextrusion process, one or more extruders feed one or more melt streams of polymer materials to a static die assembly having two or more die orifices that are adjacently arranged such that a layer extruded through one orifice merges with a layer extruded through an adjacent orifice and the layers bond together while still in the molten state. In more complex static coextrusion processes, the die assembly has multiple passages for splitting melt streams into a plurality of separate streams that are then recombined in a manner dictated by the configuration of the die assembly passages. In any event, the multilayer extrudate may be deformed into a desired configuration, and is then cooled to solidify the layers.
The above-described static coextrusion process typically is used, in one form or another, for production of multilayer articles having a relatively small number of layers, for example, from two to ten layers. Because each layer is produced by a separate die orifice or passage, the layer thickness generally must be relatively large (typically tens of microns at the exit of the die orifice). The splitting and recombining of melt streams require space and time to accomplish. Accordingly, the static coextrusion process is not suitable for producing articles having a large number of very thin layers or “microlayers”.
It has been recognized that articles manufactured to have a large number of microlayers can achieve useful properties that statically coextruded articles cannot. The number of layers can range from tens to thousands. By “microlayer” is meant a layer having a thickness in the micron or submicron range. For example, microlayers have been created in the nanometer (10−9 m) range and even in the picometer (10−12 m) range. Microlayered films have been produced having advantageous oxygen and/or moisture vapor barrier performance, advantageous absorption characteristics, advantageous tear-resistance, and advantageous optical properties, to name just a few examples.
The manufacture of microlayered articles of tubular form conventionally is accomplished through a “dynamic” coextrusion process in which two or more melt streams of the same or different polymer materials are extruded through an annular orifice of a die assembly that has one or more movable components for producing a shearing action of one melt stream relative to another. Typically the die assembly includes an inner member disposed within a bore of an outer member to form an annular passage between them, and the inner and outer members are rotatable relative to each other to shear the polymer stream being advanced along the annular passage. The various melt streams are separately fed into the annular passage and they blend together and are sheared by the rotation of the member(s), thereby producing a multiplicity of layers of the different polymer materials. The blended polymer melt stream is then discharged from an annular exit orifice of the die assembly to produce a tubular article. The rotation of the die member(s) generally is accomplished by a motor coupled through suitable gears or the like to the rotatable member(s). Often these dynamic die assemblies are relatively complex in construction, tend to have reliability problems, and can be prone to leaking.
The manufacture of microlayered articles of flat form conventionally is accomplished though a “static” coextrusion process in which two or more melt streams of the same or different polymer materials are extruded through a slit orifice of a die. Typically the die assembly includes an inner member disposed within the two halves of an outer member to form multiple slit passages between them, and additional inner members increase the number of layers by splitting the stream, stretching the split streams and recombining the split streams into one channel as the layers are being advanced along the slit passage. The various melt streams are separately fed into the slit passages, thereby producing a multiplicity of layers of the same or different polymer materials. The blended polymer melt stream is then discharged from a slit exit orifice of the die assembly to produce a flat article. Often these static die assemblies are relatively complex in construction and are fixed to a certain number of layers per assembly.